This invention relates generally to a method of determining rock properties and, more particularly, to a method that utilizes a mathematical model of a drill bit to determine the rock properties.
Identifying rock properties is key for the drilling industry and can potentially provide substantial economic benefits if performed properly and timely. Typically, rock properties are determined in the drilling industry by the use of two main methods. One of the main methods is core sampling testing, while the other main method is wireline log interpretation.
Core sampling testing is the most accurate of the two methods because the measurements are done on real rock. However, as is well known in the industry, this method is very expensive and time consuming; thereby, making it unfeasible to core the entire well. Hence, the data obtained does not provide a continuum of rock properties throughout the depth of the well. As a result, many potential economic benefits remain unrealized, such as the identification of depleted zones that are capable of producing gas. Additionally, due to the limits inherent to coring, partial or total losses of core material can occur due to jamming, failure of the core catcher, and crumbling of loose sections.
In the second alternative method, wireline logs provide measurement readings of gamma ray, sonic, resistivity, neutron, photoelectric, and density. These wireline logs are computed using specific software programs to determine firstly the type of rocks and then using special algorithms to determine the rock properties. Typically, the rock properties are identified through engineering analysis well after the well has been drilled and the drilling equipment has been disassembled. From these wireline logs, potential abnormalities may be identified, including but not limited to, overbalanced conditions, bit balling or dulling, stabilizer or BHA hang-up, stress on borehole, inadequate bit selection, hard rock, and depleted zones. However, the current methods are not capable of identifying precisely which abnormality is occurring. Additionally, the identification of potential depleted zones that are capable of producing gas are typically delayed until after all the drilling equipment has been disassembled and moved on to the next well. Once the drilling equipment has been disassembled and moved on, it is oftentimes too costly to bring the drilling equipment back to the well. Moreover, since it is not possible to precisely identify which abnormality is occurring during the well drilling, oftentimes, the drill bit may be prematurely removed from the well, which results in costly downtime.
According to some known methods, one such rock property that is measured is the rock strength, which is measured by its compressive strength. The knowledge of the rock strength has been found to be important in the proper selection and operation of drilling equipment. For example, the rock strength, for the most part, determines what type of drill bit to utilize and what weight on bit (“WOB”) and rotational speeds (“RPM”) to utilize. Rock strength may be estimated from wireline log readings using various mathematical modeling techniques. FIG. 1 shows a graph illustrating the rock properties, more particularly the unconfined compressive strength (“UCS”) of the rock, which may be read directly from sonic travel time wireline log readings. According to FIG. 1, the rock strength is inversely proportional to the sonic travel time. Thus, as the rock strength decreases, the sonic travel time increases.
FIG. 2 shows a graph illustrating the rock properties, more particularly the unconfined compressive strength of the rock, which may be read using porosity values estimated from the interpretation of the wireline logs. As seen in FIG. 2, the effective porosity—UCS relationship is roughly exponential with slight differences occurring between rocks other than sandstone. According to FIG. 2, the rock strength is inversely proportional to the effective porosity. Thus, as the rock strength decreases, the effective porosity increases. Sonic and/or acoustic impedance have even a better curve fit; however, account must again be taken for sandstone. Sandstone is known to be very light for its strength, thereby causing inaccurate interpretation of the wireline logs at times.
As known to those of ordinary skill in the art, softer rock should always be drilled at a higher rate of penetration (“ROP”) when utilizing the same drilling parameters. However, due to the rock properties of certain rocks, current methods in determining the rock strength do not provide accurate information in discerning the actual type of rock. For example, with sandstone having an acoustic impedance value of 14, it is almost impossible to drill with a medium grade bit. However, with the same acoustic impedance value for shale or carbonates, it is possible to drill with a polycrystalline diamond cutter (“PDC”) bit.
In view of the foregoing discussion, need is apparent in the art for improving methods for more accurately identifying rock properties. Further, need is apparent in the art for improving methods for more accurately identifying rock porosity. Additionally, a need is apparent for properly identifying potential abnormalities while drilling. Further, a need is apparent for properly identifying depleted zones while drilling. Furthermore, a need is apparent for properly identifying hard rock while drilling. Moreover, a need is apparent for properly identifying problems associated with the bit and other drilling tools while drilling. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit down hole drilling, for example identifying depleted zones while drilling and/or creating boreholes more effectively and more profitably. This technology is included within the current invention.